As I recall from replacing a couple of engine heads, the reason is to ensure that excess pressure isnt placed in any one place leading to warping, and to ensure an even distribution of pressure onto the gasket.

Not sure how this is relevent to Wheel Hubs...

Im not helping am I.

 

With engine heads manufacturers specify order in which screws need to be tightened. And also usually bolts need to be tightened progressively in order not to have one place with excess force just as you said.
It would be ideal if you could tighten all head bolts at the same time
And actually there is equipment that is able to tighten lots of bolts at the same time in a controlled fashion, but it is quite expensive and I doubt that anyone is using that kind of things on cars

With head bolts you usually have a specified torque to get the bolt to near its material yield point and then an additional angle in order get it past the yield point to obtain high clamping forces. Once you get to the yield point things become very non linear and the tightening torque which torque wrenches see is not accurately representing how the bolt is stretching. (Mechanics with years of experience who nearly have torque wrenches build in their hands often claim that they actually feel how such bolts are stretching when they are tightening them)

Since you are asking these bolts to stretch beyond the point where the deformation is reversible the manufacturers usually specify that you should not re-use such bolts more than certain amount of times and that you should measure how long they are before reusing them or just throw used ones away and use new ones.

Knowing all this stuff makes the wheel nut procedure bizarre. Probably you don't want to deform your drive shaft when you are replacing wheel hubs so you are not tightening the nut beyond the yield point of the drive shaft. And what difference does it make if you tighten the nut to some torque, then turn it 45 degrees and then tighten it to the final torque?

If German engineers write down such elaborate procedures the procedures may be bit overrated but knowing Italian engineers, I doubt that they write down more information that is actually needed

I also asked people with mechanical engineering degrees and car mechanics why is this needed, but no one seems to have a clue... That makes me want to know what was the guy who wrote this down thinking

 

Maybe I'm being cynical - The only explanation I can think of:

Man walks into AR dealer, looking quite angry, and approaches customer services (CS):

Man - 'My wheel fell off after fitting a new wheel hub you supplied. I'm a competent mechanic, in the trade for 30 years, and this is the first time I've had this failure - I demand satisfaction!'

CS - 'Very sorry to hear this sir, that’s most unlike our (Non electrical) parts - One assumes you followed the instructions to the letter'

Man - 'Instructions, pah, I've done this countless times... Who needs instructions for something so simple'

CS - 'So you didn’t
1. Tighten the nut to a torque 14.3 -15.8 daNm
2. Undo the nut by 90 degrees
3. Pre-tighten the nut to to 6.7 - 7.4 daNm
4. Tighten the nut by 45 degrees
5. Tighten the nut to 20 - 38 daNm'

Man - 'Course not, that’s crazy....'

CS - 'Terribly sorry sir, this is clearly a case of not fitting an item to spec... You have no case. Cheerio'.

Back on topic – Its not to allow the driveshaft to correctly bed into the hub is it? I.e., make sure the splines are correctly... nah, doesn’t make much sense either.

 

Its more feasible that following this procedure ensures that

The bearing in the hub is seated correctly
The centre hub if fitted in to the bearing correctly
Drive shaft is aligned in the hub correctly.

Have ensured the components are fitted correctly can you then get a true torque.

 

The 159 front wheel bearings are integrated in the hub and you can't take out the bearing non destructively (Maybe SKF can do it, but I don't think that ordinary people can )

So the bearing is seated and aligned in to the hub in the factory long before your car sees it.
If the drive shaft is not aligned with the hub then the hub will not want to go onto the drive shaft in the first place.

So the only sensible explanation is that the this procedure somehow ensures that the hub gets properly seated onto the drive shaft.

But in that case I would expect something like:
1. Tighten the nut to some torque
2. Tighten the nut through some angle
3. While doing this, ensure that the torque falls in some range (because if it doesn't then the hub is probably having difficulty going on to the driveshaft)

I have asked two friends with a degree in mechanical engineering who had no clue what all that procedure does. Also I asked one car mechanic who repairs high end Japanese cars and he said that he never done something like this and he also never heard that something similar needed to be done..

At the end of the day a lot of mechanics just hammer on the nut with their favourite impact wrench when taking hubs of driveshafts and when putting them back together even if they are not even remotely supposed to do that since the bearing certainly does not like the hammering action.... but who am I to judge...

Anyway... there is no problem in following the procedure right down to the last letter. Even if it had extra steps:
2.1. Have a coffee
4.1. Spin yourself around for 360 degrees

But the point is that eLearn being the official Alfa Romeo documentation, someone must have written that down for a reason and I don't like to blindly follow such instructions with no clue what each step does, but instead I like to learn new things from the Italian car masters

I am tempted to agree with Hoggins though... I guess that if you ask AR dealers how many warranty replacements they had for 159 front wheel hubs that the answer will be 0 and all failures will get attributed to "Incorrect assembly procedure"

 

On mass assembly there is still no guarantee they are assembled 100% to tolerance due to limits and fits. But this procedure as I stated will fit the assembly as required.

You could have done both in the time you have been contemplating this.

 

The hub with bad bearing on my car was replaced long time ago. I also had coffee...

I am contemplating this only because I would like to learn something new. As I said, I don't like following instructions for which I don't have explanation... Not because I think the instructions are senseless, but because I want to learn.

 

The main problem with thread engineering is that tightening torques are mostly a measure of friction. In this type of set up around 85% of what you are measuring is the thread friction. So 'Angle Controlled' assembly is what this is about.

Stage one gets the whole thing seated.
Stage two gets any swarf out and settles friction irregularity.
Stage three sets an initial bite but could still suffer from friction affects.
Stage four adds a known clamping load.
Stage five sets the thread well into its elastic range.

A curious feature of threads is that 'creep' occurs after set up. If you measure the torque to undo the thread then measure again 24 hours later it will be less.

 

Now I can finally stop contemplating about this

In between I also found a person who explained things very similar to what Old Engineer has written.
With one more additional note. My friend said that he would have to look at the whole assembly to be sure, but step 2 could also be there to control the "creep". Since tightening, undoing and tightening again is one method of reducing "creep" effects. But this is usually done when "soft" things such as gaskets are part of the joint.